The 2D models of Chapter 1 introduced many important concepts. However, we must now step away from this simplification and consider the whole three-dimensional situation. In this chapter, we analyze connections between objects in 3D. Of course, it is well known that a free body has six degrees of freedom: three independent translational degrees of freedom (often referred to as X, Y, and Z) and three independent rotational degrees of freedom (often referred to as θ_x, θ_y, and θ_z).